The present invention relates to a compound having inhibitory action against the brassinosteroid biosynthesis and a plant growth regulator comprising said compound.
Brassinosteroids have recently recognized as a new class of plant hormones through the combination of molecular genetics and researches on biosyntheses (Yokota, Trends in Plant Sci., 2, pp.137-143, 1997). Since the chemistry of brassinosteroids was established, biological activities of these homologues have been extensively studied, and their notable actions on plant growth have been revealed, which include elongation of stalks, growth of pollen tubes, inclination of leaves, opening of leaves, suppression of roots, activation of proton pump (Mandava and Annu. Rev. Plant Physiol. Plant Mol. Biol., 39, pp.23-52, 1988), acceleration of ethylene production (Schlagnhaufer et al., Physiol. Plant, 61, pp.555-558, 1984), differentiation of vessel elements (Iwasaki et al., Plant Cell Physiol., 32, pp.1007-1014, 1991; Yamamoto et al., Plant Cell Physiol., 38, pp.980-983, 1997), and cell extension (Azpiroz et al., Plant Cell, 10, pp.219-230, 1998).
Furthermore, mechanisms and regulations of physiological actions of brassinosteroids have been being revealed by variety of studies on their biosynthesis (Clouse, Plant J. 10, pp.1-8, 1996; Fujioka et al., Physiol. Plant, 100, pp.710-715, 1997). At present, 40 or more brassinosteroids have been identified. Most of C28-brassinosteroids are common vegetable sterols, and they are considered to be biosynthesized from campesterol which has the same carbon side chain as that of brassinolide.
Some Arabidopsis mutants which show characteristic dwarfism have been isolated, i.e., dwf1: Feldman et al., Science, 243, pp.1351-1354, 1989; dim: Takahashi et al., Genes Dev., 9, pp.97-107, 1995; cbb1: Kauschmann et al., Plant J., 9, pp.701-703, 1996. Their structural photomorphogenesis and dwarfism (cpd; Szekeres et al., Cell, 85, pp.171-182, 1997) and de-etiolation (det2: Li et al., Science, 272, pp.398-401, 1996; Fujioka et al., Plant Cell, 9, pp.1951-1962, 1997) are known. The mutants have deficiencies in the brassinosteroid biosynthetic pathway. Further, a dwarf mutant of Pisum sativum was recently characterized, and the mutant was reported as a brassinosteroid deficient mutant (Nomura et al., Plant Physiol., 113, pp.31-37, 1997). In these plants, use of brassinolide is known to negate severe dwarfism of the mutants. Although these findings suggest that roles of brassinosteroids are indispensable for growth and development of plants, an effective tool other than the analysis of mutants has been desired to elucidate physiological importance of brassinolide.
As seen in researches of gibberellin action, specific inhibitors against the biosynthesis are generally very effective tools for elucidating physiological functions of endogenous substances. Specific inhibitors for the brassinosteroid biosynthesis are expected to provide a new tool for understanding the functions of brassinosteroids. Uniconazol is a potent plant growth regulator (PGR) which inhibits the oxidation employed by cytochrome P-450 in the steps of the gibberellin biosynthesis from ent-kaurene to ent-kaurenoic acid. Yokota et al. observed slight reduction of the amount of endogenous castasterone as a side effect of that compound (Yokota et al., xe2x80x9cGibberellinxe2x80x9d, Springer Verlag, New York, pp.339-349, 1991). Although uniconazole inhibits differentiation of vessel elements induced by brassinolide (Iwasaki et al., Plant Cell Physiol., 32, pp.1007-1014, 1991), its inhibitory action against brassinolide is considered to be no more than an incidental action, because uniconazol essentially inhibits the gibberellin biosynthesis.
An object of the present invention is to provide a specific inhibitor against the brassinosteroid biosynthesis. Some mutants which are deficient in enzymes for biosynthesis are known for Arabidopsis, and their morphologic changes are unique to mutants with deficiency in the brassinosteroid biosynthesis. Therefore, the inventors of the present invention conducted intensive search for a compound inducing the morphologic changes unique to the mutants with the brassinosteroid biosynthesis deficiency to find a specific inhibitor against the brassinosteroid biosynthesis. As a result, they found that triazole compounds such as 4-(4-chlorophenyl)-2-phenyl-3-(1,2,4-triazoyl)butan-2-ol had the desired inhibitory action (the specification and claims of Japanese Patent Application No. 10-227939). The inventor further conducted researches, and found that the triazole compounds represented by the following formula (I) had more potent activities. The present invention was achieved on the basis of these findings.
The present invention thus provides a compound represented by the following formula (I): 
wherein R1 represents a lower alkenyl group or a phenyl group which may be substituted, R2 and R3 independently represent a phenyl group which may be substituted, or a salt thereof. According to a preferred embodiment of the present invention, provided is the aforementioned compound or a salt thereof wherein R1 is vinyl group, allyl group, butenyl group, or phenyl group, and R2 is a phenyl group which may be substituted with a halogen atom, and R3 is p-chlorophenyl group.
From another aspect of the present invention, there are provided an inhibitor against the brassinosteroid biosynthesis which comprises the compound represented by the aforementioned formula (I) or a physiologically acceptable salt thereof. The inhibitor of the present invention can be used as a plant growth regulator for, for example, suppression of plant elongation, suppression of pollen growth, retention of freshness of flowers, anti-stress agents for plants, weeds control, suppression of plant retrogradation, hypertrophism of roots and the like.
According to further aspects of the present invention, there are provided a use of the compound represented by the aforementioned formula (I) or a salt thereof for the inhibitor against the brassinosteroid biosynthesis, a method for inhibiting brassinosteroid biosynthesis which comprises the step of administering the compound of the aforementioned formula (I) or a salt thereof to a plant; and a method for regulating plant growth, which comprises the step of administering the compound of the aforementioned formula (I) or a salt thereof to a plant.